1. Field of the Invention
The present invention relates to a chuck which is used to force a semiconductor wafer to take a prescribed shape and the process of using it. The chuck allows optical monitoring of the residual air space between the rear surface of the wafer and the front surface of the chuck using an interferometric optical system.
2. Prior Art
Current chucks used to flatten wafers in metrology systems or in stepper machines have no way of monitoring if the chuck actually causes the wafer to conform exactly to the chuck surface without leaving an air space. Previously this error was ignored since the thickness variations in silicon wafers could be as high as 25 microns. The development of AccuFlat.TM. wafers by Hughes Danbury Optical Systems, Inc., Danbury, Conn., in which the wafer surfaces are highly parallel requires improved chucks which can pull down a wafer so as to leave an air space much less than the wafer thickness tolerance of 0.1 micron.
Chucks used to shape a wafer to a prescribed form must be capable of operating in either air or vacuum and these requirements have lead to the commercial development of vacuum chucks as well as electrostatic chucks. Both vacuum chucks and electrostatic wafer chucks which are commercially available have opaque bodies and do not allow any measurement of how well the wafer is shaped to the chuck surface.
Vacuum chucks initially used waffle iron or bed of nails patterns on the chuck surface in an attempt to reduce the area of the chuck in contact with the wafer. This was done to minimize the trapping of dust particles between the wafer and the chuck surface. These types of designs suffer from print through of the chuck surface pattern onto the top surface of the wafer. This was not a severe problem before the availability of AccuFlat.TM. wafers since the older wafers had huge thickness variations on the order of tens of microns.
Electrostatic chucks are large area capacitors in which the electrodes can either be a single plane at high voltage separated from the grounded wafer by an insulating layer such as Kapton.TM., manufactured by DuPont or a multiple type where alternate metal electrodes are maintained at two different high potentials (plus and minus several hundred volts) and separated from the wafer by an insulating layer. In the latter case the wafer does not require deliberate grounding to be part of the electrostatic circuit.